1. Field of the Invention
This invention relates to a method of manufacturing a liquid crystal display device and, more particularly, to a method of manufacturing a liquid crystal display device by sealing liquid crystal between the substrates relying upon a drop-injection method.
2. Description of the Related Art
A liquid crystal display device has a liquid crystal display panel constituted by two pieces of substrates and liquid crystal sealed between the two substrates. The liquid crystal display device displays a desired image by applying a voltage across the two substrates by utilizing electric and optical anisotropy of the liquid crystal molecules. It has been known that the optical characteristics of the liquid crystal display device are strongly affected by the refractive index anisotropy Δn of liquid crystal and by the cell thickness (distance between the two pieces of substrates) d.
A liquid crystal display device of the type of active matrix having a switching element for each pixel is formed through the steps described below. FIG. 11 illustrates conventional steps of manufacturing a liquid crystal display device. First, on one glass substrate, there are formed a plurality of bus lines for deciding the pixel regions, thin-film transistors (TFTs) that work as switching elements, and the like. Thereafter, an alignment film is applied onto the whole surface to prepare a TFT substrate. Onto the other glass substrate, there are formed a light-shielding film (BM) for shielding the light at the ends of the pixel regions, color filters (CFs), and the like. Thereafter, an alignment film is applied onto the whole surface to prepare a CF substrate. Next, a sealing material (adhesive) 104 is applied (see FIG. 1D) to the periphery of either one of the substrates in a manner that a portion thereof is opened to form a liquid crystal injection port 106 (see FIG. 1D). Thereafter, the two substrates are stuck together as shown in FIG. 11A to prepare a stuck substrate 100. Referring next to FIG. 11B, the stuck substrate 100 is cut to remove extra portions, and is divided into a plurality of panels 102. Referring next to FIG. 11C, the liquid crystal injection port 106 of the panel 102 is immersed in the liquid crystal 108 filled in a liquid crystal dish 110 in vacuum, and the pressure is returned back to the atmospheric pressure, so that the liquid crystal 108 is injected due to the pressure differential. After the liquid crystal 108 has been charged, a sealing material 112 such as a photo-curable resin or the like is applied as shown in FIG. 11D to seal the liquid crystal injection port 106 thereby to prepare a liquid crystal display panel 102′.
When the above liquid crystal injection method (vacuum injection method) is employed, however, the time for injecting the liquid crystal 108 increases in proportion to the square power of the diagonal size of the panel 102. In the liquid crystal display device of the VA (vertically aligned) mode in which the liquid crystal molecules are aligned perpendicularly to the surfaces of the substrates, in particular, the liquid crystal molecules are injected perpendicularly to the surfaces of the substrates requiring a liquid crystal injection time which is about three times as long as that required by the horizontally aligned liquid crystal display device. Accompanying an increase in the size of the substrates, therefore, it has been urged to shorten the time for injecting the liquid crystal.
The drop-injection method is to solve the above problems and its process comprises the following constitution. First, a sealing material 104 is applied to the circumference (whole periphery) of one substrate. Next, liquid crystals 108 are dropped in a predetermined amount onto the substrate or onto the other substrate by using a dispenser. Thereafter, the two pieces of substrates are stuck together in vacuum, and the pressure is returned back to the atmospheric pressure to inject the liquid crystals 108. The drop-injection method makes it possible to stick the substrates, to inject the liquid crystals 108 and to seal them nearly simultaneously, i.e., makes it possible to inject the liquid crystals 108 in a very short period of time as compared to that of the vacuum injection method.
However, the drop-injection method involves some of the problems including the one which is related to forming a cell thickness that strongly affects the optical characteristics of the liquid crystal display device. This stems from a difference in the process parameter for determining the cell thickness depending upon a difference in the injection system. In injecting the liquid crystal 108 relying upon the vacuum injection method, the liquid crystal injection time, the mechanical strength of the gap-support members (pillar spacers) for maintaining the cell thickness, the height thereof and the density of arrangement thereof serve as parameters for determining the cell thickness. When the liquid crystals 108 are injected relying upon the drop-injection method, on the other hand, the volume in the cell determined by the surface shapes of the two substrates to be stuck together and the amount of dropping the liquid crystals 108 serve as parameters for determining the cell thickness. Therefore, the cell thickness is calculated from the volume in the cell and the amount of dropping the liquid crystals 108. If the height of the pillar spacers is too large relative to the cell thickness, bubbles evolve in the liquid crystal display panel. If the height of the pillar spacers is too small, on the other hand, the cell thickness becomes locally irregular in the liquid crystal display panel due to distortion at the time of sticking the substrates together.
Table 1 shows examples of the state in the liquid crystal display panel depending upon the cell thickness and the density of arrangement of the pillar spacers. Here, it is presumed that the pillar spaces are formed all having a uniform height on the substrate surface so as to obtain a cell thickness of 4 μm, and that the cell thickness varies depending upon the dropping amount of the liquid crystals 108. Further, each pixel has a size of about 100 μm×300 μm.
TABLE 1CellDroppingthick-amountnessfluc-Density of pillar spacer arrangement(μm)tuationOne per 6 pixelsOne per 12 pixelsOne per 24 pixels3.72−7%Bubbles evolvedBubbles evolvedBubbles evolved3.76−6%Bubbles evolvedBubbles evolvedBubbles evolved3.80−5%Bubbles evolvedBubbles evolvedGood3.84−4%Bubbles evolvedBubbles evolvedGood3.88−3%Bubbles evolvedGoodGood3.92−2%GoodGoodGood3.96−1%GoodGoodGood4.000GoodGoodGood4.04+1%GoodGoodGood4.08+2%GoodGoodGood4.12+3%Cell thicknessGoodGoodlocally irregular4.16+4%Cell thicknessCell thicknessGoodlocally irregularlocally irregular4.20+5%Cell thicknessCell thicknessGoodlocally irregularlocally irregular4.24+6%Cell thicknessCell thicknessCell thicknesslocally irregularlocally irregularlocally irregular4.28+7%Cell thicknessCell thicknessCell thicknesslocally irregularlocally irregularlocally irregular
In the liquid crystal display panel in which the pillar spacers are arranged in a number of one per 6 pixels as shown in Table 1, bubbles evolve in the liquid crystal display panel as the cell thickness becomes smaller than 3.92 μm, and the thickness of the liquid crystal display panel becomes locally irregular as the cell thickness becomes greater than 4.08 μm. Therefore, the cell thickness in a range of 3.92 to 4.08 μm, i.e., the dropping amount fluctuation of liquid crystal 108 in a range of ±2% becomes a production margin for the cell thickness. In the liquid crystal display panel in which the pillar spacers are arranged in a number of one per 12 pixels, bubbles evolve in the liquid crystal display panel as the cell thickness becomes smaller than 3.88 μm, and the thickness of the liquid crystal display panel becomes locally irregular as the cell thickness becomes greater than 4.12 μm. Therefore, the cell thickness in a range of 3.88 to 4.12 μm, i.e., the dropping amount fluctuation of liquid crystals 108 in a range of ±3% becomes a production margin for the cell thickness. In the liquid crystal display panel in which the pillar spacers are arranged in a number of one per 24 pixels, bubbles evolve in the liquid crystal display panel as the cell thickness becomes smaller than 3.80 μm, and the thickness of the liquid crystal display panel becomes locally irregular as the cell thickness becomes greater than 4.20 μm. Therefore, the cell thickness in a range of 3.80 to 4.20 μm, i.e., the dropping amount fluctuation of liquid crystals 108 in a range of ±5% becomes a production margin for the cell thickness.
As described above, the production margin of the cell thickness increases with a decrease in the density of arranging the pillar spacers. However, if the density of arranging the pillar spacers is too low, the thickness of the cells tend to be varied when pushed by fingers or the like. In many cases, therefore, forming the pillar spacers in a number of one per 24 pixels is a limit of density of arrangement. The production margin of the cell thickness in this case is in a range of ±5%.
This is based on the assumption that the pillar spacers on the same substrate are all formed maintaining a uniform height. In practice, however, the pillar spacers include a fluctuation of about ±0.15 μm in their height. In the liquid crystal display panel having a cell thickness of 4 μm, this corresponds to about ±3% to 4%. Therefore, the real production margin of the cell thickness becomes smaller than ±5%.
In the step of injecting the liquid crystals based on the drop-injection method, therefore, it becomes important to drop the liquid crystals 108 in a suitable amount to improve the yield of production while suppressing the evolution of bubbles and local irregularity of cell thickness. Therefore, there has been proposed a method of determining the amount of dropping the liquid crystals 108 based on the height of the pillar spacers (see, for example, JP-A-2001-281678). The dropping amount can be correctly determined if the height of the pillar spacers is measured just prior to dropping the liquid crystals 108. For this purpose, however, an additional step is necessary for measuring the height of the pillar spacers prior to dropping the liquid crystals 108 resulting in an increase in the tact (time required by an apparatus for treating a piece of substrate). In general, the pillar spacers are measured for their height in their whole number or in part of their number immediately after a layer is applied on the whole surface of the substrate to form pillar spacers or immediately after the pillar spacers are formed by the subsequent patterning to guarantee the stability of the process, e.g., so that a change in time series thereof can be comprehended. By utilizing the result of this measurement, therefore, the steps of production can be simplified rather than measuring again the height of the pillar spacers prior to dropping the liquid crystals 108, and the cost of production can be lowered.
It was, however, found that depending upon the materials forming the pillar spacers, the height of the pillar spacers after formed undergoes a change through the treatments described below.    (1) The height of the pillar spacers decreases if the substrate surfaces are subjected to the reforming treatment such as ashing for improving the printing property at the time of applying an alignment film on the substrate.    (2) The height of the pillar spacers decreases if the substrate is regenerated by the alignment film regeneration treatment by peeling off the alignment film that was irregularly applied and forming again the alignment film. This is due to that the pillar spacers formed by using an organic material such as an acrylic resin, a novolak resin or a polyimide resin are partly dissolved by an NMP (N-methylpyrrolidone) or a TMAH (tetramethylammonium hydroxide) aqueous solution used for peeling off the alignment film formed by using a polyimide resin or the like.    (3) The height of the pillar spacers decreases if the substrate is heat-treated (annealed) at a high temperature (e.g., not lower than 160° C.) because the pillar spacers formed by using an organic polymer are thermally deformed (thermally distorted).
Among them, the treatments (1) and (3) can be set to be executed the same number of times for all substrates in a lot. Therefore, a decrease in the height of the pillar spacers can be set to be nearly the same for all substrates in the lot. If the decrease in the height of the pillar spacers is nearly the same for all substrates, the amount of dropping the liquid crystals can be determined based on a difference between the height of the pillar spacers measured immediately after the formation of the pillar spacers and the amount of decrease in the height thereof. However, the alignment film regeneration treatment of (2) above may not be executed even once or may be executed repetitively a plurality of number of times depending upon the substrate. Therefore, the amount of decrease in the height of the pillar spacers differs depending upon the substrates, and it is difficult to determine a suitable dropping amount of liquid crystals based on the height of the pillar spacers measured immediately after the formation of the pillar spacers.
FIG. 12 illustrates the amount of decrease in the height of the pillar spacers depending upon the number of times of regenerating the alignment film, wherein the abscissa represents the number of times of regenerating the alignment film and the ordinate represents the amount of decrease (μm) in the height of the pillar spacers. Referring to FIG. 12, the amount of decrease in the height of the pillar spacers is nearly proportional to the number of times of regenerating the alignment film. The height of the pillar spacers decreases by about 0.03 μm after every regeneration of the alignment film. This corresponds to 0.75% of the cell thickness when the liquid crystal display panel has the cell thickness of 4 μm. Therefore, if the dropping amount of the liquid crystals 108 is determined based on the height of the pillar spacers measured just after they are formed, a problem arouses in that the production margin for the cell thickness is deviated if the treatment for regenerating the alignment film is executed a plurality of number of times.